Han skrattar högt när jag skämtar.

skrattar is the present tense of the verb skratta (to laugh). Swedish present tense doesn’t change for person/number (no laugh/laughs difference like in English).
So you get:

• jag skrattar = I laugh / I’m laughing
• han skrattar = he laughs / he’s laughing
• de skrattar = they laugh / they’re laughing

Either can be correct in English depending on context. Swedish often uses the simple present where English might choose either:

• Han skrattar högt = He laughs loudly / He is laughing loudly
  Swedish can be more explicit with an “ongoing” sense (e.g. håller på att skratta), but it’s not necessary here.

Because högt is the adverb form meaning loudly. In Swedish, many adjectives add -t when used as adverbs (especially when modifying verbs):

• Han skrattar högt = He laughs loudly
  Compare adjective use:
• en hög röst = a loud voice
• ett högt ljud = a loud sound

Because the sentence doesn’t have to state who you’re joking with; it just says when I joke (i.e., when I make jokes / crack jokes). If you want to specify the target or partner, you can:

• ... när jag skämtar med honom = when I joke with him
• ... när jag skämtar med dig = when I joke with you
• ... när jag skämtar om det = when I joke about it

skämta can cover both:

• skämta = to joke / to kid / to joke around
  Context decides whether it’s “tell jokes” or “joke around.”
  If you specifically mean “tell a joke” as a noun, Swedish often uses:
• dra ett skämt = tell a joke (literally “pull a joke”)
  But jag skämtar is very natural for “I’m joking.”

Yes.

• när is used for something real/recurring or a known time:
  Han skrattar högt när jag skämtar = He laughs loudly when I joke (habitually / whenever that happens).
• om is used more for a future/conditional “if/when” idea:
  Han skrattar om jag skämtar can sound more like “He laughs if I joke” (more conditional).

It’s a normal subordinate clause introduced by när. In Swedish subordinate clauses, the basic order is: subject + verb (and if there’s a negation like inte, it usually comes before the verb):

• när jag skämtar = when I joke
• när jag inte skämtar = when I don’t joke

Yes, and then Swedish triggers inversion (verb comes before subject in the main clause):

• När jag skämtar skrattar han högt.
  Literally: When I joke, laughs he loudly.
  Meaning: When I joke, he laughs loudly.

Swedish punctuation is flexible here. A comma before a subordinate clause is often optional in short sentences:

• Han skrattar högt när jag skämtar. (common)
  You can add a comma for clarity/emphasis, especially in longer sentences:
• Han skrattar högt, när jag skämtar. (possible, but can feel a bit heavy in a short sentence)

Key points (approximate guidance):

• skrattar: the sk before r is pronounced like sk in “ski” (not “school”). The tt is a crisp t sound.
• högt: ö is like the vowel in German schön (not in English). The gt is often pronounced with a k sound at the end (can sound like hökt in many accents).
• skämtar: sk before ä is a “sh” sound: sh-. ä is like the vowel in “cat” for many speakers (but often a bit more open).

högt is the most direct and idiomatic for “loudly” with laughing.

• mycket means “a lot/very much,” so han skrattar mycket = he laughs a lot (frequency/amount, not volume).
• starkt can mean “strongly” and is used more with sounds like stark musik (loud music) or tala starkt, but for laughing högt is the standard choice.

Sometimes, but it changes the feel.

• när = when (general/whenever)
• då = then/when (often points to a specific moment, and it’s common in spoken Swedish)
  You might hear:
• Han skrattar högt då jag skämtar (possible, a bit more formal/old-fashioned with då
  • clause)
    More commonly, då is used like “then”:
• Jag skämtar, och då skrattar han högt. = I joke, and then he laughs loudly.